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Dana Mill
Known Participant
August 13, 2022
Answered

Histogramme luminosité et RVB

  • August 13, 2022
  • 3 replies
  • 724 views

Bonjour, je dois envoyer des photos à un imprimeur pour un livre et je veux être sûr de bien les préparer. Il ne me donne comme consigne que de les mettre au format sRVB.  Dans Photoshop, à quel histogramme dois-je me fier pour être sûr d'avoir une exposition correcte : luminosité ou RVB ? Car ils sont nettement différents. Souvent l'histogramme luminosité est correct: les noirs ne sont pas bouchés et les blancs ne sont pas brûlés, alors que l'histogramme RVB indique une sous-exposition (le plus souvent), d'une ou plusieurs couleurs. Merci.

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Correct answer TheDigitalDog

The Histogram tells you nothing about actual exposure if it's from a raw. No Adobe software provides a raw Histogram. Your last screen capture shows the color, not luminance (that's what one Histogram mode is called even though it's not luminance). Again, the video explains the differences. 

3 replies

TheDigitalDog
TheDigitalDog
Inspiring
August 13, 2022

This may help with your question(s) about brightness, saturation etc (which has nothing to do with Exposure):

 

Everything you thought you wanted to know about Histograms
Another exhaustive 40-minute video examining:
What are histograms? In Photoshop, ACR, Lightroom.
Histograms: clipping color and tones, color spaces, and color gamut.
Histogram and Photoshop’s Level’s command.
Histograms don’t tell us our images are good (examples).
Misconceptions about histograms. How they lie.
Histograms and Expose To The Right (ETTR).
Are histograms useful and if so, how?

Low rez (YouTube): http://www.youtube.com/watch?v=EjPsP4HhHhE
High rez: http://digitaldog.net/files/Histogram_Video.mov

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"
Stephen Marsh
Community Expert
Stephen MarshCommunity Expert
Community Expert
August 13, 2022

The histogram is good for info on the overall image, however, I would suggest that you check or set the tonal range with the info panel. I use a colour sample tool with say a 3x3 or higher average in the lightest and darkest areas of the image that contain detail and then set them to something like 245rgb and 10rgb (presuming neutral values) via levels or curves.

 

https://helpx.adobe.com/photoshop/using/making-quick-tonal-adjustments.html

 

https://jkost.com/blog/2018/03/the-eyedropper-color-samplers-and-info-panel-in-photoshop-cc.html

 

https://www.ledet.com/margulis/Makeready/MA21-Defanging.pdf

 

https://www.ledet.com/margulis/PP5E_Ch03_r1.pdf

 

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    August 13, 2022
    quote

    presuming neutral values


    By @Stephen Marsh

     

    Generally I agree with Stephen, but in this case I suspect the problem is saturation rather than tonal values. In other words, gamut clipping. The attached example looks like a classic case of low blue clipping.

     

    The histogram is IMO underrated as a diagnostic tool. It doesn't say anything about how the image looks - but if something's wrong, it can often tell you precisely what in a single glance.

     

    The most important thing to watch is not overall value distribution, but how the channels are distributed relative to each other. That's very often the smoking gun.

      D Fosse
      Community Expert
      D FosseCommunity Expert
      Community Expert
      August 13, 2022

      You can't trust either, they don't show the full story.

       

      This is the histogram view you should use:

       

      And I suspect this is how your channels are actually distributed. Note the low end clipping in the blue channel. This is gamut clipping. It is not, as you might think, caused by the blue sky. It is caused by the deep yellows in the shaded foliage. The blue channel "wants" to go lower to represent this very deep yellow, but can't. So it clips.

       

      The RGB histogram just averages out the channels. The luminance histogram weights the channels in a way that approaches the Lab L channel (long story). Either way, they don't give you the full picture.

        D
        Dana MillAuthor
        Known Participant
        August 13, 2022

        Do you mean that the luminance histogram doesn't show if there are details in the shadows and the highlights? That's my concern. Actually the colors histogram looks like this. It's much more exposed to the left than the luminance histogram, for blue, green and red layers as well. Does'it mean that this photo is underexposed, even though the luminance histogram is not? Thank you. 

        D
        Dana MillAuthor
        Known Participant
        August 14, 2022

        The Histogram tells you nothing about actual exposure if it's from a raw. No Adobe software provides a raw Histogram. Your last screen capture shows the color, not luminance (that's what one Histogram mode is called even though it's not luminance). Again, the video explains the differences. 


        I watched the video and I understand that the histogram that reflects the exposure is the luminosity histo, not the RGB. Thank you for this very helpfull video. 

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